Zoom lens system

ABSTRACT

A zoom lens system includes, in order from an object side to an image side, first to fourth lens groups respectively having positive, negative, positive, and positive refractive powers. When zooming from a wide-angle end to a telephoto end, the first and third lens groups move to the object side and the second lens group moves to the image side, whereby a spacing between the first and second lens groups and a spacing between the third and fourth lens groups are increased and a spacing between the second and third lens groups is decreased. The fourth lens group moves for focusing. The following condition is satisfied: 
             0   &lt;            MG   ⁢           ⁢     2   ·   Y         f   ⁢           ⁢   2            ≤   2.3         
where MG 2  represents the movement range of the second lens group when zooming from the wide-angle end to the telephoto end, Y represents a maximum diagonal length of the image plane, and f 2  represents the focal length of the second lens group.

CLAIM OF PRIORITY

This application claims priority to Taiwanese Patent Application No. 097116025 filed on Apr. 30, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system, and particularly to a compact zoom lens system with a short overall length and a high image resolution for forming a real image on a digital or non-digital image pickup device of a camera.

2. Description of Prior Art

In recent years, with the rapid integration of optical technology into digital electronic technology, most electronic devices such as camera mobile phones, small digital cameras and small video cameras have been integrated with a zoom lens system for effecting the photographic function. Such a zoom lens system is generally required to be small in size and light in weight for portability, while being capable of providing a high zoom ratio and a high image resolution. Generally, a high zoom ratio lens system consists of a plurality of lens groups and a considerable number of constituent lenses, whereby the overall length of the lens system is rather long. To meet the additional high-resolution requirement, special low dispersion lenses and aspheric lenses are further included in the lens system.

With the development of semiconductor technology, aspheric lenses have been widely used in a photographic lens system. The adoption of an aspheric lens effectively corrects spherical aberration associated with spherical lenses. Further, an aspheric lens functions equivalent to several spherical lenses, whereby the cost can be reduced and a compact lens system can be ensured. For a zoom lens system, in order to obtain the desired zoom ratio and optical performance within the entire zoom range while ensuring a compact configuration, the lens configuration for each lens group of the zoom lens system must be carefully designed. Conventionally, a photographic zoom lens system generally employs three lens groups in a negative-positive-positive refractive power configuration, wherein two of the three lens groups are movable for realizing zooming. However, the movement ranges of the two movable lens groups are relatively large, and the imaging performance dramatically varies with the increase of zoom ratio. To overcome these disadvantages, a zoom lens system consisting of four lens groups and thus an increased number of constituent lenses has been introduced. Three of the four lens groups are generally configured to be movable for realizing zooming. However, in order to achieve a high zoom ratio of 5× and a high image resolution, a long overall length of such a zoom lens system is necessitated for allowing movement of the three lens groups. When all the four lens groups are configured to be movable, both the number of constituent lenses and the manufacturing cost of the zoom lens system are generally increased. Further, whether three or four lens groups are configured to be movable for effecting zooming, the outer diameter of each lens group of the conventional four-group zoom lens system is generally large. This results in a large retraction space and a large retraction length when the conventional four-group zoom lens system is retracted.

It is known in the zoom lens art that, to reduce the overall length of a zoom lens system, a direct and efficient solution is to reduce the movement range of each lens group during zooming. Unfortunately, this solution generally requires an increase in the manufacturing precision and difficulty of the zoom lens system and a reliable aberration correction effect is also difficult to be ensured. Therefore, how to simplify the lens configuration so as to reduce the size and weight of the whole zoom lens system while maintaining high optical performance including a high zoom ratio and a high image resolution is a difficult problem encountered by a zoom lens designer.

Hence, an improved four-group zoom lens system, which is compact in size and short in retraction length while providing a high zoom ratio and a high image resolution, is desired to overcome the above problems encountered in the prior art.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a zoom lens system consisting of four lens groups in a positive-negative-positive-positive refractive power configuration to provide a high zoom ratio and a high image resolution, the zoom lens system also being short in the overall length and small in size.

To achieve the above object, the present invention provides a zoom lens system including, in order from an object side to an image side along an optical axis thereof, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power. When zooming from a wide-angle end to a telephoto end, the first and third lens groups move to the object side and the second lens group moves to the image side. The zoom lens system of the present invention satisfies the following condition:

$0 < {\frac{{MG}\;{2 \cdot Y}}{f\; 2}} \leq 2.3$ where MG2 represents the movement range of the second lens group when zooming from the wide-angle end to the telephoto end, Y represents a maximum diagonal length of the image plane, and f2 represents the focal length of the second lens group.

The fourth lens group moves to effect the focusing operation, so that a shift in the image plane due to a variation in magnification during zooming is compensated.

The second lens group comprises, in order from the object side to the image side along the optical axis of the zoom lens system, a meniscus concave lens, a biconvex lens and a meniscus concave lens, wherein the biconvex lens has at least one aspheric surface.

The zoom lens system of the present invention also satisfies the following condition:

$0.60 \leq \frac{{MG}\;{1 \cdot {fw}}}{{fT} \cdot Y} \leq 1.00$ where MG1 represents the movement range of the first lens group when zooming from the wide-angle end to the telephoto end, fw represents the focal length of the zoom lens system of the present invention at the wide-angle end, fT represents the focal length of the zoom lens system at the telephoto end, and Y, as mentioned above, represents the maximum diagonal length of the image plane.

The first lens group comprises, in order from the object side to the image side along the optical axis of the zoom lens system, a meniscus concave lens and a meniscus convex lens.

The zoom lens system of the present invention further satisfies the following condition:

$1.06 \leq \frac{f\;{3 \cdot {MG}}\; 3}{f\; 1} \leq 2.06$ where f3 represents the focal length of the third lens group, MG3 represents the movement range of the third lens group when zooming from the wide-angle end to the telephoto end, and f1 represents the focal length of the first lens group.

The third lens group serves to compensate spherical and coma aberrations. The third lens group comprises, in order from the object side to the image side along the optical axis of the zoom lens system, a biconvex lens having at least one aspheric surface, and a cemented lens constructed by a biconvex lens cemented with a biconcave lens.

The fourth lens group moves to effect focusing. When focusing, the fourth lens group moves toward the object side to approach the third lens group, whereby the spacing between the third and fourth lens groups is decreased. This movement of the fourth lens group serves to compensate a shift in the image plane due to a variation in magnification during zooming. The fourth lens group also may serve as a compensating lens to move together with the second and third lens groups during the zooming operation, and then move independently for effecting the focusing operation. The fourth lens group comprises a biconvex lens.

The zoom lens system of the present invention further includes an aperture stop disposed between the second and third lens groups. The aperture stop moves with the third lens group during zooming.

The zoom lens system of the present invention comprises four lens groups in a positive-negative-positive-positive refractive power configuration. During the zooming operation, both the first and third lens groups move toward the object side, and the second lens group moves toward the image side. The fourth lens group moves to effect the focusing operation. By the employment of three movable lens groups for realizing zooming and the incorporation of aspheric lenses, the zoom lens system of the present invention provides a high zoom ratio of as high as 5× and a high image resolution. During zooming, the movement range of the second lens group is relatively small. This effectively reduces the outer diameter of the second lens group and thus increases the possibility of obtaining a compact zoom lens system when retracted. The configuration of the zoom lens system of the present invention also helps to reduce the outer diameters of both the second and third lens groups, whereby the retraction space for receiving the zoom lens system is correspondingly reduced. Thus, a compact zoom lens system is obtained, which is short in retraction length, small in outer diameter and high in zoom ratio (5 times). Further, the zoom lens system of the present invention consists of only nine constituent lenses, which results in a short overall length and a reduced manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be best understood through the following description with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view illustrating the construction of a zoom lens system according to the concept of the present invention;

FIGS. 2A-2C are respective graphic representations of field curvature according to Numerical Embodiment 1 of the present invention at a wide-angle end, an intermediate position and a telephoto end;

FIGS. 3A-3C are respective graphic representations of distortion aberration according to Numerical Embodiment 1 of the present invention at the wide-angle end, the intermediate position and the telephoto end;

FIGS. 4A-4C are respective graphic representations of lateral color according to Numerical Embodiment 1 of the present invention at the wide-angle end, the intermediate position and the telephoto end;

FIGS. 5A-5C are respective graphic representations of longitudinal spherical aberration according to Numerical Embodiment 1 of the present invention at the wide-angle end, the intermediate position and the telephoto end;

FIGS. 6A-6C are respective graphic representations of coma aberration according to Numerical Embodiment 1 of the present invention at the wide-angle end;

FIGS. 6D-6F are respective graphic representations of coma aberration according to Numerical Embodiment 1 of the present invention at the intermediate position;

FIGS. 6G-6I are respective graphic representations of coma aberration according to Numerical Embodiment 1 of the present invention at the telephoto end;

FIGS. 7A-7C are respective graphic representations of field curvature according to Numerical Embodiment 2 of the present invention at the wide-angle end, the intermediate position and the telephoto end;

FIGS. 8A-8C are respective graphic representations of distortion aberration according to Numerical Embodiment 2 of the present invention at the wide-angle end, the intermediate position and the telephoto end;

FIGS. 9A-9C are respective graphic representations of lateral color according to Numerical Embodiment 2 of the present invention at the wide-angle end, the intermediate position and the telephoto end;

FIGS. 10A-10C are respective graphic representations of longitudinal spherical aberration according to Numerical Embodiment 2 of the present invention at the wide-angle end, the intermediate position and the telephoto end;

FIGS. 11A-11C are respective graphic representations of coma aberration according to Numerical Embodiment 2 of the present invention at the wide-angle end;

FIGS. 11D-11F are respective graphic representations of coma aberration according to Numerical Embodiment 2 of the present invention at the intermediate position; and

FIGS. 11G-11I are respective graphic representations of coma aberration according to Numerical Embodiment 2 of the present invention at the telephoto end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above-mentioned and other technical contents, features and effects of the present invention will become apparent from the hereinafter set forth detailed description of preferred numerical embodiments of the present invention in combination with the drawings.

The present invention provides a zoom lens system, which is used in an image pickup device or a photographic device for forming an image of an object onto an image sensor (CCD or CMOS). The lens construction of the zoom lens system in accordance with the present invention is illustrated in FIG. 1, in which reference numeral 10 denotes an optical axis of the zoom lens system in accordance with the present invention, reference numeral 12 denotes an image plane, symbol “OBJ” denotes an object side, symbol “IMA” denotes an image side, symbol “W” denotes a wide-angle end, symbol “M” denotes an intermediate position, symbol “T” denotes a telephoto end, and symbol “EG” schematically denotes a glass element such as a low-pass filter or a cover glass on the image sensor. The arrows shown in FIG. 1 indicate the moving directions of the constituent lens of the zoom lens system of the present invention when the zoom lens system is zoomed from the wide-angle end (W), through the intermediate position (M), toward the telephoto end (T).

As shown in FIG. 1, the zoom lens system of the present invention includes, in order from the object side OBJ to the image side IMA along the optical axis 10 thereof, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, a third lens group G3 having positive refractive power, an aperture stop ST disposed between the second and third lens groups G2, G3, a fourth lens group G4 having positive refractive power, and a glass element EG disposed proximate to the image plane 12.

The first lens group G1 comprises, in order from the object side OBJ to the image side IMA along the optical axis 10, a meniscus concave lens L1 having negative refractive power and a meniscus convex lens L2 having positive refractive power. The second lens group G2 comprises, in order from the object side OBJ to the image side IMA along the optical axis 10, a meniscus concave lens L3 having negative refractive power, a biconvex lens L4 having positive refractive power and a meniscus concave lens L5 having negative refractive power, wherein the biconvex lens L4 has at least one aspheric surface. The third lens group G3 serves to compensate spherical and coma aberrations. The third lens group G3 comprises, in order from the object side OBJ to the image side IMA along the optical axis 10, a biconvex lens L6 having positive refractive power and a cemented lens. The biconvex lens L6 has at least one aspheric surface, and the cemented lens is constructed by a biconvex lens L7 having positive refractive power cemented with a biconcave lens L8 having negative refractive power. The fourth lens group G4 comprises a biconvex lens L9 having positive refractive power. The above constituent lenses of the zoom lens system of the present invention may be all made of glass. Alternatively, some constituent lenses such as aspheric lenses L4, L6 an L9 may be made of plastics.

When zooming from the wide-angle end to the telephoto end, as shown in FIG. 1, the first and third lens groups G1, G3 of the zoom lens system of the present invention move toward the object side OBJ, while the second lens group G2 moves toward the image side IMA, so that the spacing D4 between the first and second lens groups G1, G2 is increased and the spacing D10 between the second and third lens groups G2, G3 is decreased. During the zooming operation, the aperture stop ST moves together with the third lens group G3.

The fourth lens group G4 moves to effect focusing. During focusing, the fourth lens group G4 moves toward the object side OBJ to approach the third lens group G3, whereby the spacing D16 between the third and fourth lens groups G3, G4 is decreased. This movement of the fourth lens group G4 serves to compensate a shift in the image plane 12 due to a variation in magnification during zooming. The fourth lens group G4 also may serve as a compensating lens to move together with the second and third lens groups G2, G3 during the zooming operation, and then move independently for effecting the focusing operation.

The movement range of the first lens group G1 of the zoom lens system of the present invention during zooming is represented by MG1 and satisfies the following condition (1):

$\begin{matrix} {0.60 \leq \frac{{MG}\;{1 \cdot {fw}}}{{fT} \cdot Y} \leq 1.00} & (1) \end{matrix}$ where MG1 represents the movement range of the first lens group G1 when zooming from the wide-angle end to the telephoto end, fw represents the focal length of the zoom lens system of the present invention at the wide-angle end, fT represents the focal length of the zoom lens system at the telephoto end, and Y represents a maximum diagonal length of the image plane 12.

The movement range of the second lens group G2 of the zoom lens system during zooming is represented by MG2. This movement range MG2 of the second lens group G2 is relatively small and satisfies the following condition (2):

$\begin{matrix} {0 < {\frac{{MG}\;{2 \cdot Y}}{f\; 2}} \leq 2.3} & (2) \end{matrix}$ where MG2 represents the movement range of the second lens group G2 when zooming from the wide-angle end to the telephoto end, Y represents the maximum diagonal length of the image plane 12, and f2 represents the focal length of the second lens group G2.

The movement range of the third lens group G3 of the zoom lens system during zooming is represented by MG3 and satisfies the following condition (3):

$\begin{matrix} {1.06 \leq \frac{f\;{3 \cdot {MG}}\; 3}{f\; 1} \leq 2.06} & (3) \end{matrix}$ where f3 represents the focal length of the third lens group G3, MG3 represents the movement range of the third lens group G3 when zooming from the wide-angle end to the telephoto end, and f1 represents the focal length of the first lens group G1.

The zoom lens system of the present invention is constructed by four lens groups G1-G4 consisting of nine lenses L1-L9. Three of the nine lenses L1-L9 are preferably configured to be aspheric lenses to correct aberrations. In detail, the biconvex lens L4 of the second lens group G2, the biconvex lens L6 of the third lens group G3 and the biconvex lens L9 of the fourth lens group G4 are aspheric lenses having at least one aspheric surface. The aspheric surfaces of these aspheric lenses are expressed by the following formula:

${DD} = {\frac{{CH}^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)C^{2}H^{2}}}} + {E_{4}H^{4}} + {E_{6}H^{6}} + {E_{8}H^{8}} + {E_{10}H^{10}} + {E_{12}H^{12}} + {E_{14}H^{14}} + {E_{16}H^{16}}}$ where DD represents displacement in the direction of the optical axis at the position of height H from the optical axis relative to the surface vertex; C=1/r; r is the curvature radius of the aspheric lens surface on the optical axis; H represents a height of a point on the aspheric surface with respect to the optical axis; K represents a cone constant; and E₄, E₆, E₈, E₁₀, E₁₂, E₁₄ and E₁₆ are respectively aspheric coefficients for fourth, sixth, eighth, tenth, twelfth, fourteenth and sixteenth order terms.

Two numerical embodiments of the zoom lens system of the present invention will be described in detail hereinafter.

Numerical Embodiment 1

FIGS. 2A-6I illustrate various aberrations generated by the zoom lens system according to Numerical Embodiment 1 of the present invention, wherein FIGS. 2A-2C are respective graphic representations of field curvature at the wide-angle end, the intermediate position and the telephoto end, FIGS. 3A-3C are respective graphic representations of distortion aberration in the three different zooming positions, FIGS. 4A-4C are respective graphic representations of lateral color in the three different zooming positions, FIGS. 5A-5C are respective graphic representations of longitudinal spherical aberration in the three different zooming positions, and FIGS. 6A-6I are respective graphic representations of coma aberration or transverse ray fan plots in the three different zooming positions. It can be seen from these graphs that the zoom lens system of Numerical Embodiment 1 of the present invention provides excellent correction of various aberrations and thus a high level of optical performance.

Numerical values of the constituent lenses of the zoom lens system according to Numerical Embodiment 1 of the present invention are shown in Data Table 1 given below. In Data Table 1 and other similar data tables provided hereinafter, “i” represents the order of the surface from the object side OBJ (including lens surfaces, the aperture stop ST and the glass element EG), “Ri” represents the radius of curvature (mm) of the ith surface, “D” represents the ith member thickness (wherein the ith member is defined as a physical element associated with the ith surface) or the distance (mm) between the ith surface and the (i+1)th surface, and “Nd” and “Vd” respectively represent the refractive index (d-line) and Abbe number (d-line) of the ith member.

DATA TABLE 1 Surface (i) Ri (mm) D (mm) Nd Vd R1 37.515 1.15 1.84666 26.78 R2 21.159 0.35 R3 22.504 4.05 1.804 52.57 R4 −933.598 D4 R5 127.088 0.85 1.8045 46.71 R6 5.61 2.7 R7 22.272 2.15 1.84 23.59 R8 −42.23 0.59 R9 −17.363 0.75 1.7 50.02 R10 −789.704 D10 R11(ST) ∞ 1.05 R12 5.186 2.3 1.59 59.11 R13 −176.784 0.1 R14 8.326 1.9 1.81 44.91 R15 −7.086 0.6 1.74 30.01 R16 4.057 D16 R17 15.159 2.4 1.58913 61.15 R18 −30.171 0.8 R19 ∞ 0.8 1.516798 64.21 R20 ∞ D20

Data Table 2 given below shows aspheric coefficients for the aspheric surfaces of the zoom lens system according to Numerical Embodiment 1, wherein K represents a cone constant and E4, E6, E8, E10, E12, E14 and E16 are respectively aspheric coefficients for fourth, sixth, eighth, tenth, twelfth, fourteenth and sixteenth order terms. It is shown in Data Table 2 that the zoom lens system according to Numerical Embodiment 1 of the present invention includes five aspheric surfaces. Specifically, with reference to FIG. 1, the object-side surface R7 of the biconvex lens L4 of the second lens group G2, both the object-side surface R12 and the image-side surface R13 of the biconvex lens L6 of the third lens group G3, and both the object-side surface R17 and the image-side surface R18 of the biconvex lens L9 of the fourth lens group G4 are configured to be aspheric surfaces.

DATA TABLE 2 Surface No. R7 R12 R13 R17 R18 K −10.96274094 0 0 0 0 E4 4.420896E−04 6.058374E−04 2.413530E−03 3.630398E−05 3.658195E−05 E6 −6.902375E−06 1.108731E−04 1.282318E−04 2.994900E−06 1.949309E−06 E8 7.624546E−07 −3.740475E−05 4.197375E−06 −7.576349E−09 −8.256693E−08 E10 −1.097260E−08 1.100017E−05 1.006265E−06 −3.351617E−09 −2.291387E−09 E12 −6.706674E−10 −9.019351E−07 8.220411E−08 −8.391009E−11 −8.575064E−11 E14 3.923479E−11 E16 −4.420989E−13

Data Table 3 provided below shows variable spacings between the four lens groups G1-G4 and between the glass element EG and the image plane 12 of the zoom lens system of the present invention at the respective wide-angle end (W), the intermediate position (M) and the telephoto end (T) according to Numerical Embodiment 1. In Data Table 3, D4 denotes a first variable spacing along the optical axis 10 between the image-side surface R4 of the first lens group G1 and the object-side surface R5 of the second lens group G2, D10 denotes a second variable spacing along the optical axis 10 between the image-side surface R10 of the second lens group G2 and the object-side surface R11 of the third lens group G3, D16 denotes a third variable spacing along the optical axis 10 between the image-side surface R16 of the third lens group G3 and the object-side surface R17 of the fourth lens group G4, D20 denotes a fourth variable spacing along the optical axis 10 between the image-side surface R20 of the glass element EG and the image plane 12, and Y denotes a maximum diagonal length of the image plane 12. In addition, the focal lengths f of the zoom lens system according to Numerical Embodiment 1 at the respective wide-angle end (W), the intermediate position (M) and the telephoto end (T) are also provided in Data Table 3.

DATA TABLE 3 Y = 4.1 mm D4 D10 D16 D20 W (f = 5.26) 0.63 12.55438536 5.085004706 1.96 M (f = 11.38) 6.200875043 4.10714222 9.19241456 2.95 T (f = 24.92) 20.39966185 0.69 12.28998343 3.06

Various parameters and their values of the zoom lens system according to Numerical Embodiment 1 are listed in the following Data Table 4, wherein fW denotes the focal length of the zoom lens system of Numerical Embodiment 1 at the wide-angle end, fT denotes the focal length of the zoom lens system of Numerical Embodiment 1 at the telephoto end, f1 denotes the focal length of the first lens group G1, f2 denotes the focal length of the second lens group G2, f3 denotes the focal length of the third lens group G3, f4 denotes the focal length of the fourth lens group G4, MG1 denotes the movement range of the first lens group G1 when zooming from the wide-angle end to the telephoto end, MG2 denotes the movement range of the second lens group G2 when zooming from the wide-angle end to the telephoto end, MG3 denotes the movement range of the third lens group G3 when zooming from the wide-angle end to the telephoto end, and Y represents a maximum diagonal length of the image plane 12. It can be obtained from Data Table 4 that, for Numerical Embodiment 1 of the present invention, the values for the above conditions (1), (2) and (3) are respectively 0.835, 1.654 and 1.578, all of which fall within the respective required ranges.

DATA TABLE 4 fW fT f1 f2 f3 f4 MG1 MG2 MG3 Y 5.26 24.92 51.6 −8.8 9.8 17.5 16.22 −3.55 8.31 4.1

Numerical Embodiment 2

FIGS. 7A-11I illustrate various aberrations generated by the zoom lens system according to Numerical Embodiment 2 of the present invention, wherein FIGS. 7A-7C are respective graphic representations of field curvature at the wide-angle end, the intermediate position and the telephoto end, FIGS. 8A-8C are respective graphic representations of distortion aberration in the three different zooming positions, FIGS. 9A-9C are respective graphic representations of lateral color in the three different zooming positions, FIGS. 10A-10C are respective graphic representations of longitudinal spherical aberration in the three different zooming positions, and FIGS. 11A-11I are respective graphic representations of coma aberration or transverse ray fan plots in the three different zooming positions. It can be seen from these graphs that the zoom lens system of Numerical Embodiment 2 of the present invention provides excellent correction of various aberrations and thus a high level of optical performance.

Numerical values of the constituent lenses of the zoom lens system according to Numerical Embodiment 2 are shown in Data Table 5 given below.

DATA TABLE 5 Surface (i) Ri (mm) D (mm) Nd Vd R1 33.916 1.15 1.84666 20.78 R2 19.922 0.35 R3 21.026 4.05 1.7725 44.00 R4 −1262.833 D4 R5 56.735 0.85 1.804 41.00 R6 6.389 2.615 R7 21.584 1.84 1.8446 18.99 R8 −39.569 0.98 R9 −10.034 0.75 1.58313 40.00 R10 −100.000 D10 R11(ST) ∞ 1.05 R12 5.313 2.3 1.58913 61.15 R13 −36.864 0.1 R14 11.214 1.9 1.7859 52.20 R15 −7.233 0.9 1.69895 32.20 R16 3.996 D16 R17 15.000 2.25 1.58913 61.15 R18 −47.111 2 R19 ∞ 5.00E−01 1.52308 58.57 R20 ∞ D20

Data Table 6 given below shows aspheric coefficients for the aspheric surfaces of the zoom lens system according to Numerical Embodiment 2. It is shown in Data Table 6 that the zoom lens system according to Numerical Embodiment 2 includes five aspheric surfaces. Specifically, with reference to FIG. 1, both the object-side surface R7 and the image-side surface R8 of the biconvex lens L4 of the second lens group G2, both the object-side surface R12 and the image-side surface R13 of the biconvex lens L6 of the third lens group G3, and the object-side surface R17 of the biconvex lens L9 of the fourth lens group G4 are configured to be aspheric surfaces.

DATA TABLE 6 Surface No. R7 R8 R12 R13 R17 K 1.190993542 0 0.138225361 0 0 E4 2.261305E−04 −4.049566E−05 −6.742856E−05 1.638594E−03 1.352637E−04 E6 3.776902E−06 −2.029148E−06 6.519552E−05 1.020305E−04 −3.345976E−06 E8 −7.702267E−07 7.952130E−09 −6.245938E−06 −2.767034E−06 5.168913E−07 E10 7.052331E−08 2.677975E−09 1.705313E−06 1.775399E−06 −1.947245E−08 E12 −2.492140E−09 1.281748E−10 −9.511725E−08 −4.595718E−08 2.707642E−10 E14 4.225225E−11

Data Table 7 provided below shows variable spacings between the four lens groups G1-G4 and between the glass element EG and the image plane 12 of the zoom lens system of the present invention at the respective wide-angle end (W), the intermediate position (M) and the telephoto end (T) according to Numerical Embodiment 2. In addition, the maximum diagonal length Y of the image plane 12 and the focal lengths f of the zoom lens system according to Numerical Embodiment 2 at the respective wide-angle end (W), the intermediate position (M) and the telephoto end (T) are also provided in Data Table 7.

DATA TABLE 7 Y = 4.1 mm D4 D10 D16 D20 W (f = 6.43) 0.63 12.55438536 5.085004706 1.59 M (f = 13.94) 6.342556678 4.060797515 9.224167829 2.84 T (f = 30.48) 20.39966116 0.69 12.28998 1.9

Various parameters and their values of the zoom lens system according to Numerical Embodiment 2 are listed in the following Data Table 8. It can be obtained from Data Table 8 that, for Numerical Embodiment 2 of the present invention, the values for the above conditions (1), (2) and (3) are respectively 0.793, 1.839 and 1.553, all of which fall within the respective required ranges.

DATA TABLE 8 fW fT f1 f2 f3 f4 MG1 MG2 MG3 Y 6.43 30.48 49.8 −9.7 10.3 19.6 15.42 −4.35 7.51 4.1

As described above, the zoom lens system of the present invention is a four-group zoom lens system having a positive-negative-positive-positive optical configuration. During the zooming operation, the first and third lens groups G1, G3 move toward the object side OBJ, while the second lens group G2 moves toward the image side IMA. The fourth lens group G4 moves to effect the focusing operation. By the employment of three movable lens groups for realizing zooming and the incorporation of aspheric lenses, the zoom lens system of the present invention provides a high zoom ratio of as high as 5× and a high image resolution. During zooming, the movement range of the second lens group G2 is relatively small. This effectively reduces the outer diameter of the second lens group G2 and thus increases the possibility of obtaining a compact zoom lens system when retracted. The configuration of the zoom lens system of the present invention also helps to reduce the outer diameters of both the second and third lens groups G2, G3, whereby the retraction space for receiving the zoom lens system of the present invention is correspondingly reduced. Thus, a compact zoom lens system is obtained, which is short in retraction length, small in outer diameter and high in zoom ratio (5 times). Further, the zoom lens system of the present invention consists of only nine constituent lenses, which results in a short overall length and a reduced manufacturing cost.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

1. A zoom lens system, in order from an object side to an image side along an optical axis thereof, comprising: a first lens group having positive refractive power; a second lens group having negative refractive power; a third lens group having positive refractive power; and a fourth lens group having positive refractive power; wherein, during zooming from a wide-angle end to a telephoto end, the first lens groups moves toward the object side and the second lens groups moves toward the image side to increase a spacing between the first and second lens groups and to decrease a spacing between the second and third lens groups; and wherein the zoom lens system satisfies the following condition: $0.60 \leq \frac{{MG}\;{1 \cdot {fw}}}{{fT} \cdot Y} \leq 1.00$ where MG1 represents movement range of the first lens group when zooming from the wide-angle end to the telephoto end, fw represents focal length of the zoom lens system at the wide-angle end, fT represents focal length of the zoom lens system at the telephoto end, and Y represents a maximum diagonal length of an image plane.
 2. The zoom lens system as claimed in claim 1, wherein the first lens group comprises, in order from the object side to the image side along the optical axis, a meniscus concave lens and a meniscus convex lens.
 3. The zoom lens system as claimed in claim 1, wherein the second lens group comprises, in order from the object side to the image side along the optical axis, a meniscus concave lens, a biconvex lens and a meniscus concave lens.
 4. The zoom lens system as claimed in claim 3, wherein the biconvex lens of the second lens group is an aspheric lens.
 5. The zoom lens system as claimed in claim 1, wherein the third lens group is a compensating lens group and comprises, in order from the object side to the image side along the optical axis, a biconvex lens and a cemented lens.
 6. The zoom lens system as claimed in claim 5, wherein the biconvex lens of the third lens group is an aspheric lens and the cemented lens is constructed by a biconvex lens cemented with a biconcave lens.
 7. The zoom lens system as claimed in claim 1, wherein the fourth lens group moves toward the object side to approach the third lens group for focusing, whereby the spacing between the third and fourth lens groups is decreased.
 8. The zoom lens system as claimed in claim 7, wherein the fourth lens group also serves as a compensating lens group to move together with the second and third lens groups for zooming, and then moves independently for focusing.
 9. The zoom lens system as claimed in claim 7, wherein the fourth lens group comprises an aspheric biconvex lens.
 10. The zoom lens system as claimed in claim 1 further comprising an aperture stop disposed between the second and third lens groups, the aperture stop moving with the third lens group during zooming.
 11. The zoom lens system as claimed in claim 1, wherein the zoom lens system satisfies the following condition: $1.06 \leq \frac{f\;{3 \cdot {MG}}\; 3}{f\; 1} \leq 2.06$ where f3 represents focal length of the third lens group, MG3 represents movement range of the third lens group when zooming from the wide-angle end to the telephoto end, and f1 represents focal length of the first lens group.
 12. The zoom lens system as claimed in claim 1, wherein the zoom lens system satisfies the following condition: $0 < {\frac{{MG}\;{2 \cdot Y}}{f\; 2}} \leq 2.3$ where MG2 represents movement range of the second lens group when zooming from the wide-angle end to the telephoto end, Y represents the maximum diagonal length of the image plane, and f2 represents the focal length of the second lens group. 